I have done school dances two ways. One, I was one of three or four local area professionals who got paid that night by the company that we did the shooting for/with. I brought my own equipment and they supplied the backgrounds and the helpers who took the money, filled out the order forms, etc. Depending on the size of the school, I would get $200 or more for a couple hours work.
I have also done this on my own, and though it is much more profitable, there are some important things to consider. Make sure you are insured. Those kids can get pretty excited with all that's going on. I had a power pack get knocked around, and my insurance company replaced it. Also, if a kid trips on your cord, or whatever, you want liability.
You will want a friend or two to help you take the money, the orders, and generally keep the line moving. You need to shoot fast, as there isn't as much time as you think to get them all shot. Have a board showing the packages where they stand in line so they can see your prices, packages, etc. Give receipts with your contact information/order number. State a delivery time and stick to it so your reputation doesn't get dissed.
You are usually the one to supply the background, and have it go with their dance theme. I have had students help me with the prep before the date of the dance, it really depends on the school.
You'll be wise to choose a professional lab that does school dance/event processing. They can print the packages in the order you shot them, etc. Always go to the lab first and ask them the best way to do it so they can accurately print each customer's package. This is important! Also, does the lab do digital as well as film? Just good to know. They may give you stickers to number your rolls/roll sheets (the forms to record each frame you take), envelopes, etc.
If you get time to shoot all the couples and go out on the dance floor, I'd be surprised. But it depends on the size of the school, and the duration of the dance, and how fast you are. I haven't dealt with that type of dance floor photography, though it does sound fun!
It is a great way to make money, and there is a lot or responsibility on your part.
I might have left a few things out, and sorry for the random order of suggestions (typing as I think of things)!
If you do get to do this, please let us know! Good luck!
There is nothing worse than a brilliant image of a fuzzy concept. - Ansel Adams
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